Prior art tubular assemblies for the transfer of hot or cold fluids use inner and outer pipes with insulation in the annular space therebetween. These assemblies are utilized in many ways, such as for the enhanced recovery of oil from an oil well by the injection of steam. The conduit sections of prior art assemblies isolate the fluid passing through their inner pipes from the ambient environment and thus provide a relatively effective thermal barrier therebetween.
However, the joints between successive conduit sections provide paths that allow a relatively high degree of heat transfer between the inner pipe, which is normally at the temperature of the fluid, and the outer pipe which is near the temperature of the ambient environment. The term "joint", as used herein, is intended to include the immediate coupling as well as those portions of each conduit section which are proximate thereto.
When fluids, which are at a high temperature and pressure relative to the ambient environment, are transferred through the conduit assembly, the joint between successive conduit sections is required not only to contain the fluid within the assembly, but must also withstand the thermal expansion and/or contraction of the inner pipe relative to the outer pipe. Still further requirements must be complied with as the situation dictates. Thus, each joint area of a multiple-section conduit assembly for the enhanced recovery of oil by steam injection must (1) thermally isolate the steam from the surrounding environment; (2) prevent leakage and maintain the steam pressure at an acceptable level; (3) allow for the expansion of the inner pipes relative to each other and relative to the cooler outer pipes; (4) support the hanging weight of substantially all the conduit sections strung vertically below the joint; and (5) be rugged enough to withstand repeated assembly and disassembly operations. Similar problems are presented when fluids are transferred at very low temperatures and pressures relative to the ambient environment.
Typically, the joints of prior art assemblies provide a poorer thermal barrier than the remainder of the conduit section through which the fluid passes. For example, the joints shown in U.S. Pat. No. 3,275,345, incorporated herein by reference, include frusto-conical members, one being affixed to the corresponding ends of the pipes at each end of a conduit section. Each pair of members supports the inner and outer pipes in a spaced, coaxial relationship and, further, it transfers the expansion and contraction forces developed by the heating and cooling of the inner pipe relative to the outer pipe. As shown in the patent, successive conduit sections are joined together by bolts passed through flanges affixed to the exterior of the outer pipe. The flanges maintain a space or gap between the inner pipes of the conduit sections. The frusto-conical members form a wedge-shaped space at the joint which communicates with the interior of the inner pipes through the aforementioned gap. The fluid in the inner pipe is allowed to fill this wedge-shaped space through the gap and come into contact with the surfaces of the frusto-conical members and the outer pipe.
Although the frusto-conical members transfer the expansion/contraction forces developed in the inner pipe to the outer pipe efficiently, a relatively poor thermal barrier exists at each joint resulting from contact of the fluid with the outer pipes, the frusto-conical members, and the flange of the joint. Thus, the thermal efficiency of the entire assembly is diminished.
Still, other prior art assemblies include joints in which two conforming frusto-conical surfaces mate in the manner of a bayonet type joint. In the latter assembly, the inner pipes of the respective conduit sections are sealed together by a liquid seal between the end surfaces of each inner pipe. An expansion mechanism is provided near the inner pipe joint to allow for the expansion/contraction of the inner pipe relative to the outer pipe. The conical surfaces of the mated pair of frusto-conical members in the aforementioned assembly are closely fitted together and sealed to each other. Hence, fluid carried by the assembly is isolated within the inner pipes and away from the outer pipes as well as from any further joining means attached thereto. The repeated assembly and disassembly of the latter prior art structure is difficult and expensive.